Lambdas are by deafult const so that they produce the same output every time they are run. This lambda needs to set the value on a captured promise which is a mutating operation, so it must be mutable.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372336 91177308-0d34-0410-b5e6-96231b3b80d8